For example, a typical silicon-based solar cell has a configuration including an antireflection film and a light-receiving surface electrode via an n+ layer on the upper surface of a silicon substrate of p-type polycrystalline semiconductor and including a back surface electrode (hereinafter simply “electrode” when no distinction is made between these electrodes) via a p+ layer on the lower surface. The antireflection film is for the purpose of reducing a surface reflectance while maintaining a sufficient visible light transmittance and is made of a thin film of silicon nitride, titanium dioxide, silicon dioxide, etc.
The light-receiving surface electrode of the solar cell is formed with a method called fire-through, for example. In this electrode forming method, for example, after the antireflection film is disposed on the entire surface of the n+ layer, a conductive paste is applied in an appropriate shape onto the antireflection film by using a screen printing method, for example, and is subjected to a firing (sintering) treatment. This method simplifies the operation as compared to the case of partially removing the antireflection film to form an electrode in the removed portion and eliminates a problem of displacement between the removed portion and the electrode forming position. The conductive paste consists mainly of, for example, a silver powder, a glass frit (flaky or powdery fragments of glass formed by melting, quenching, and, if needed, crushing a glass raw material), an organic vehicle, and an organic solvent and, since a glass component in the conductive paste breaks the antireflection film in the course of the firing, an ohmic contact is formed between the conductive component in the conductive paste and the n+ layer (see, e.g., Patent Document 1).
Various proposals have hitherto been made in such a solar cell light-receiving surface electrode formation for a purpose such as enhancing the fire-through property to improve the ohmic contact and consequently to increase a fill factor (FF) and energy conversion efficiency. For example, the group five elements such as phosphorus are added to the conductive paste to promote the oxidation-reduction effect of glass and silver to the antireflection film, improving the fire-through property (see, e.g., Patent Document 1 above). Chloride, bromide, or fluoride is added to the conductive paste to assist the effect of glass and silver breaking the antireflection film with these additives, improving the ohmic contact (see, e.g., Patent Document 2).
It is also proposed for a silver-containing paste containing 85 to 99 (wt %) of silver and 1 to 15 (wt %) of glass that the glass has a composition containing 15 to 75 (mol %) of PbO and 5 to 50 (mol %) of SiO2 and not containing B2O3 (see, e.g., Patent Document 4). This silver-containing paste is used for the solar cell electrode formation and the ohmic contact is considered to be improved by using the glass having the composition described above.
A thick film conductive composition is also proposed that contains a silver powder, a zinc-containing additive, and a glass frit having a softening point within a range of 300 to 600 (degrees C.) dispersed in an organic solvent (see, e.g., Patent Document 5). This thick film conductive composition is for the purpose of forming a light-receiving surface electrode of a solar cell, and the conductivity and the solder adherence property are improved by adding zinc.
A conductive paste for a solar cell element is also proposed that contains glass frit containing zinc oxide and lead oxide within ranges of 40 to 70 (wt %) and 1 to 10 (wt %), respectively, and a conductive material such as silver (see, e.g., Patent Document 6). Since this paste can ensure an adhesive strength without coating an electrode surface with solder etc., a highly reliable electrode layer can be produced with high productivity.